Skull Confirms Older Origin for 'Living Fossil' Fish

An artist's depiction of the fish that left behind a fossil skull, which has confirmed that modern-looking coelacanths have been around for more than 400 million years. CREDIT: Brian Choo View full size image

A group of ancient fish, called coelacanths, have changed so little over time they are known as "living fossils." Now, the remains of a skull found in the Yunnan Province of China, confirms these creatures have been around, largely unchanged, for more than 400 million years.

Once thought to have died out at roughly the time the dinosaurs disappeared, the first living coelacanth was discovered in a fishing net in 1938 off the eastern coast of South Africa. Since then, others have turned up elsewhere along the coasts of the Indian Ocean. [Image Gallery: Freaky Fish]

While it's clear their history goes way back, the fossils they left behind have been scarce so far. A lower jawbone, more than 400 million years old and discovered in Australia, hinted at the earliest known emergence of coelacanths whose appearance matched the two species alive today. This small fossil has been described as the oldest coelacanth, but the authors of the recent research write that it offers so little information that it cannot be reliably placed within the fishes' family tree.

The newest fossil evidence, the remains of a skull, date back to almost the same time and contain more definitive features that indicate both it and the Australian fossil were "modern" coelacanths, according to the study researchers writing in the April 10 issue of the journal Nature Communications.

The discovery reinforces what was already suspected about coelacanths: After a period of rapid diversification long ago, these fish have remained pretty much the same over hundreds of millions of years, according to Matt Friedman, a lecturer in paleobiology at the University of Oxford, who was not involved in the research.

"It makes their sort of anatomical conservation, this lack of major change over geological time, much more impressive," Friedman said.

In fact, the discovery extends the record for coelacanths with modern-looking bodies back by about 17 million years, according to the researchers, led by Min Zhu of the Chinese Academy of Sciences.

Coelacanths belong to an ancient group, the lobe-finned fishes, which have fins attached to stalks, rather than directly to their bodies. More than 400 million years ago, the coelacanths are believed to have split from off from other lobe-finned fishes, which later gave rise to modern lung fishes and to tetrapods — four-footed animals, including us. 

So, our closest relatives among fishes are lung fish and the living species of coelacanths, which belong to the genus Latimeria, Friedman said.

Modern coelacanths are set apart by their symmetrical tail, a relatively long snout and a jointed skull, which allows them to open their mouths widely. More primitive lobe-finned fish also have this joint; however, their skulls are proportioned differently, according to Friedman.

The skull joint and the presence and position of large sensory pores — part of a system fish use to detect changes in water pressure — helped the researchers place the skull within the coelacanths' family tree. 

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